1. Field of the Invention
This invention relates to an ultrasmall magnetic recording and reproducing apparatus, such as a still video floppy device in an electronic still camera. More particularly, it relates to the centering of a magnetic disk in any such apparatus.
2. Description of the Prior Art
Referring to FIG. 3, there is shown a disk cartridge which is used in a magnetic recording and reproducing apparatus of the type to which this invention pertains. It comprises a casing 1 having an upper portion 1a and a lower portion 1b which are formed from a hard synthetic resin. A flexible magnetic disk 2 is rotatably housed in the casing 1. The upper and lower portions 1a and 1b of the casing are formed with a window 3 which can be closed by a shutter 4 fitted slidably on the casing 1 and having a substantially channel-shaped cross section. The cartridge also has a central hub 5 formed from a synthetic resin and defining the center of rotation of the disk 2. The upper and lower casing portions 1a and 1b are centrally formed with an opening 6. The hub 5 is rotatable about the center of the opening 6. The hub 5 has a hole 7 in which a part of a magnetic recording and reproducing apparatus can be inserted as will hereinafter be described, and a thin elastic wall 5a formed adjacent to the hole 7. An annular yoke 8 is fitted about the hub 5 as shown in FIG. 4, and the disk 2 is bonded to the yoke 8. Thus, the disk 2, the hub 5 and the yoke 8 form a unitary part.
The disk cartridge is mounted on the driving system of a magnetic recording and reproducing apparatus as shown in FIG. 4. The driving system includes a motor which rotates the disk 2.
The apparatus shown in FIG. 4 includes a base 9 to which a fixed shaft 10 is secured by press fitting at one end thereof. Ball bearings 11 and 12 surround the shaft 10 in a vertically spaced apart relationship from each other and support a rotary driving member 13 rotatably about the shaft 10. A turntable 14 is secured to the driving member 13 and provided with an annular driving magnet 15 on its underside. An insulating plate 16 is secured to the base 9 and carries a driving coil 17 facing the driving magnet 15. The rotary driving member 13, the turntable 14, the magnet 15 and the coil 17 form the driving motor so that the turntable 14 may be rotated about the shaft 10.
A rotor yoke 18 is secured to the lower end of the rotary driving member 13 and an annular FG magnet 19 is provided on the underside of the yoke 18. The base 9 carries a frequency signal generator plate (FG plate) 20 facing the FG magnet 19. If the FG magnet 19 is rotated, a change occurs to a magnetic field and causes the FG plate 20 to generate an inverse electromotive force which is outputted as a frequency signal for controlling the rotation of the driving motor.
The turntable 14 has an upstanding annular wall 14a and an annular magnet 21 is provided within the wall 14a. The magnet 21 is provided for attracting the yoke 8 and securing it on the wall 14a so that the disk 2 may be rotated with the turntable 14. A guide member 22 is attached to the upper end of the shaft 10 and has an upwardly tapered surface 22a.
If the disk cartridge is mounted on the driving system in the magnetic recording and reproducing apparatus, the shutter 4 is opened by a projection provided on the apparatus, though not shown, and exposes a part of the disk 2 through the window 3. If the cartridge is, then, lowered, its casing 1 is positioned in place by a positioning pin not shown. The hub 5 is fitted about the rotary driving member 13 as its hole 7 is guided by the tapered surface 22a of the guide member 22, while the yoke 8 is attracted by the magnet 21 and rests on the turntable 14. The peripheral wall of the hole 7 is held by the elastic wall 5a against the outer peripheral surface of the rotary driving member 13 at three points thereof, whereby the hub 5 is centered relative to the shaft 10. The yoke 8 abuts on the upper end of the wall 14a and is thereby positioned axially of the shaft 10, whereby the disk 2 is positioned in place.
If an electric current is supplied to the coil 17 through a circuit not shown to rotate the rotary driving member 13 and the turntable 14, the disk 2 is also rotated and a desired picture signal is recorded on the disk 2 by a magnetic head not shown, but brought into contact with the disk 2 through the window 3 of the casing 1.
The apparatus as hereinabove described has a number of drawbacks as will hereunder be pointed out:
(1) As the upper end of the rotary driving member 13 is inserted in the hole 7 of the hub 5, the disk 2 fails to be positioned accurately if the thickness of the rotary driving member 13 between its outer surface 13a and the outer ring of the upper ball bearing 11 is not uniform.
(2) Even if the surface 13a may be formed with a uniform outside diameter, it is likely to lose uniformity, resulting in the failure of the disk to be accurately centered, as the ball bearing 11 is press fitted into the upper end of the rotary driving member 13.
(3) A high degree of surface finish accuracy is required of the surface 13a with which the peripheral wall of the hole 7 of the hub 5 is kept in contact when it is lowered. If the surface 13a has a low degree of surface finish accuracy, it damages the hub 5 and disables it to be centered accurately.
(4) A high degree of hardness is required of the surface 13a for the same reason as that stated at (3) above. If it has only a low degree of hardness, it easily gets worn and disables the hub to be accurately centered.
(5) As the surface 13a is located radially outwardly of the ball bearing 11, the outside diameter of the ball bearing 11 is greatly limited. It is necessary to use a small and expensive ball bearing.
In order to avoid these drawbacks, it has been usual to work the rotary driving member 13 with a high degree of accuracy, harden, plate or otherwise treat the outer surface of the rotary driving member 13, or grind the surface 13a again after the ball bearing 11 has been fitted in the rotary driving member 13. All of these methods have, however, the disadvantage of lowering the efficiency of apparatus production and raising its cost.